The devices that operate in so-called microwave frequency bands typically use microwave filters. Among the microwave filters, there are notably filters of rejection or “band-stop” type, the function of which is to reject signals with a frequency contained in a determined frequency band, as well as so-called “band-pass” filters, that allow only signals with a frequency contained in a determined frequency band to pass.
The microwave filters may comprise planar transmission lines and resonators formed by discrete components such as self-inductances and capacitors. The microwave filters are constrained by the tolerances of the elements from which they are made, notably the thickness of the substrate on which the transmission lines are produced, the permittivity and the permeability of the substrate, as well as by the performance tolerance levels of the discrete components used. The variability of all the abovementioned parameters can lead to inadequate manufacturing efficiencies or to performance levels that are overall too random, more particularly in the following cases:                when the microwave filters have one or more frequency bands cut at low frequency, situated in a passband that is overall relatively wide, this first case being illustrated by FIG. 1 described in detail hereinbelow;        when the microwave filters are incorporated in multilayer substrate structures, notably in the case where the filters are incorporated in a monolithic subsystem also comprising a large number of elements. In such a case, a filter whose performance levels are situated outside of the desired specifications means scrapping the complete subsystem, and therefore reduced manufacturing efficiency. When a plurality of microwave filters are incorporated in one and the same module, the reduction in manufacturing efficiency is all the more critical;        when the microwave filters comprise vias. Such a case occurs in particular when the microwave filters comprise resonators with an end that is short-circuited to a ground, as is the case for the microwave filters that are the subject of the present invention;        when the filters are compact microwave filters produced on substrates with high permittivity and/or permeability, particularly sensitive to the production tolerances and to the electrical parameters such as the permittivity and permeability;        when the microwave filters are used in systems for which it is necessary to perform an adjustment of the filter in its application context;        when the microwave filters form multiplexers.        
A major problem in the context of the design of microwave filters arises when the stopbands are situated at relatively low frequencies compared to the highest frequencies that the microwave filter has to allow to pass, that is to say the high cutoff frequency of the overall passband of the filter. Hereinbelow, the term “fundamental resonance frequency” will be used to designate the first resonance frequency of a microwave resonator around which the stopband is situated in the case of a band-stop filter, or, similarly, the passband in the case of a band-pass filter, the subsequent resonance frequencies determining the overall passband of the filter.
In order to produce a microwave filter, for example of rejection type, that has a cut frequency band that is narrow and at relatively low frequency, in a passband that is globally wide, it is possible, according to techniques that are known per se, to produce the microwave filter by means of a so-called “mixed” technology, that is to say on the one hand with localized elements, typically capacitors and/or self-inductances, and on the other hand distributed elements: typically coupled parallel lines, as is illustrated by FIG. 4, described in detail hereinbelow. The self-inductances and capacitors used may be components of “SMC” type, SMC standing for Surface-Mount Component. The self-inductances of SMC type that are available typically have resonance frequencies, quality coefficients and tolerances that are inadequate. Also, to a lesser extent, the capacitors of SMC type typically present the same drawbacks. The self-inductances that take the form of air coils offer better performance levels than their monolithic peers of SMC type, but present problems linked to implementation difficulty, in other words mounting and placement that are difficult, as well as performance problems linked to microphony, that is to say phenomena whereby vibrations of the structure can lead to a displacement of the turns of the coil, and consequently the generation by the latter of spurious signals.
The performance levels of such mixed structures are further limited in the field of high frequencies, notably by the localized components. Moreover, the tolerances of these components and their implementation introduce significant spreads in the performance levels of the microwave filter. These spreads limit the performance levels thereof and can result in inadequate manufacturing efficiencies.
According to another technique that is known per se, the microwave filters can be produced without discrete localized elements such as self-inductances or SMC capacitors. According to this technique, the microwave filters may comprise so-called impedance jump resonators, commonly referred to by the acronym SIR, standing for “Stepped Impedance Resonator”. Such resonators typically exhibit resonance frequencies higher than the fundamental resonance frequency, differing by multiples of this fundamental frequency. Such resonators are illustrated by FIG. 7, described in detail hereinbelow.
A so-called “invariant” resonator, that is to say with no characteristic impedance jump, made up of a so-called “half-wave” line section, that is to say a line section delimited by two short circuits or by two open circuits, has a fundamental resonance frequency f0, and higher resonance frequencies equal to the multiples of the fundamental resonance frequency F0, i.e. 2F0, 3F0, etc., as is illustrated by FIG. 5, described hereinbelow.
A resonator of invariant type made up of a single so-called “quarter-wave” line section, that is to say a line section delimited by a short circuit and an open circuit, has a first resonance frequency f0, and higher resonance frequencies equal to the odd multiples of the first resonance frequency F0, i.e. 3F0, 5F0 , etc., as is illustrated by FIG. 6, described hereinbelow. Each of the higher resonance frequencies is reflected in “replicas” of the fundamental response, that is to say stray passbands or stopbands, depending on the type of response of the filter.
An SIR resonator of so-called “quarter-wave” type with two sections as illustrated by FIG. 7 makes it possible to separate the first resonance frequency f0 and the second resonance frequency denoted Fres2. The second resonance frequency is then typically much higher than 3f0. The second resonance frequency becomes all the higher as the characteristic impedance ratio of the two sections of the resonator increases.
However, the planar line technologies exhibit producible minimum and maximum characteristic impedance limits which limit the ratio between the second resonance frequency and the first resonance frequency Fres2/F0, and consequently the passband of the microwave filter, denoted BPG.
Furthermore, the SIR resonators are sensitive to the manufacturing tolerances and to the tolerances of the materials used.